1. Field of the Invention
The present invention concerns communication from one integrated circuitry chip to another, and more particularly concerns supplying a more nearly optimal reference voltage for receiving data.
2. Related Art
In a chip-to-chip communication interface within a computer system data is sent from a driver on one chip to a receiver on another chip. In order for such an interface to operate efficiently at high data rates, the receiving chip must have an accurate reference voltage. The optimal reference voltage is dependent on many factors, including the supply voltage at the driver on one chip, the supply voltage at the receiver of the other chip and the amount of noise on the data nets in between, among others.
Referring now to FIG. 1, certain aspects are illustrated for a conventional chip-to-chip communication interface 100, according to the prior art. In the conventional interface a static voltage divider circuit 114 is used to generate a reference voltage 115 for a first chip, that is, chip A 110 on a first printed circuit board 101. Similarly, a static voltage divider circuit 124 is used to generate a reference voltage for chip B 120 on another printed circuit board 102.
This arrangement has a number of problems. First, the optimal reference voltage for a receiver will change if the supply voltage changes for the driver. For example, the desired reference voltage VREFB 125 for receivers 122 on chip 120 will change if the supply voltage V SUPPLYA 111 changes for drivers 113 on chip 110. However, according to the conventional interface illustrated the reference voltage VREFB 125 generated by voltage divider 124 only depends on the supply voltage V SUPPLYB 121 at the chip 120 of the receivers 122. This is commonly problematic because often a board will be released with a particular chip, such as a processor for example, and then later a pin-compatible, upgraded version of the processor will become available, typically with a lower supply voltage.
Also, in the arrangement depicted here the chips are on two different circuit boards. It is problematic in this sort of arrangement that the supply voltage V SUPPLYA 111 of the chip 110 drivers 113 exists only on the circuit board 101 where the chip 110 resides and not on the chip 120 having the receivers 122, for example. This is an obstacle to the receivers 122 which depend on the supply voltage V SUPPLYA 111 of the drivers 113.
Furthermore, as data signals propagate from a driver such as one of drivers 113 to a receiver such as one of the receivers 122 the signals acquire noise based on their surroundings, so that the signals received differ from the signals sent. Ideally, the reference voltage VREFB 125 at a receiver 122 should vary depending upon the amount of noise that a signal has acquired since being sent by the driver 113, so that the receiver can cancel out the noise and detect the signals that were actually sent. However, the static voltage divider 124 does not see the same noise.
For at least the above reasons a need exists for an improved method and apparatus for supplying a reference voltage.